GD-421: Add uid path support to Scene Runner

[mpewsey]

Is your feature request related to a problem? Please describe.
Currently the Scene Runner does not support loading scenes by uid:// paths.

If a uid:// path is used, the GdUnitSceneRunner: Can't load scene by given resource path... branch from the following snippet of the Scene Runner initializer is executed:

# handle scene loading by resource path
	if typeof(p_scene) == TYPE_STRING:
		if !FileAccess.file_exists(p_scene):
			if not p_hide_push_errors:
                                # *** uid:// paths fall into this error ***
				push_error("GdUnitSceneRunner: Can't load scene by given resource path: '%s'. The resource not exists." % p_scene)
			return
		if !str(p_scene).ends_with("tscn"):
			if not p_hide_push_errors:
				push_error("GdUnitSceneRunner: The given resource: '%s'. is not a scene." % p_scene)
			return
		_current_scene = load(p_scene).instantiate()
		_scene_auto_free = true

Describe the solution you'd like
I would like to add support for uid:// paths to the Scene Runner initializer. These paths can be preferable over res:// paths since it is possible to rename project files without invalidating hard coded path references.

Describe alternatives you've considered
In GDScript, to get around this limitation, you can load the scene resource via the uid:// path, then pass that to the initializer instead. However, in C#, the ISceneRunner currently only accepts a string path as an argument. Therefore, adding uid:// path support would be especially useful for C#.

In C#, you can get around this issue by doing the following, but this requires some extra steps:

var path = "uid://cn8ucy2rheu0f"
var scene = ResourceLoader.Load<PackedScene>(path);
var runner = ISceneRunner.Load(scene.ResourcePath);

Additional context

I am willing to implement this feature and will submit a follow up pull request.

[MikeSchulze]

Thank you for submitting this feature request.
I was not aware that there was this feature to load scenes by UID ;)